1. Field of the Invention
The present invention relates to a liquid crystal display unit including a sequence circuit that is supplied plural supply voltages and supplies plural supply voltages to a liquid crystal display panel in a predetermined sequence, and a liquid crystal TV set that includes the sequence circuit.
2. Description of the Related Art
In each of liquid crystal display units and liquid crystal TV sets, plural types of voltages are supplied to its liquid crystal display panel through a predetermined cable. The liquid crystal display panel makes normal operations with those types of voltages supplied in a predetermined sequence. Consequently, supply and stop of the supply of those plurality types of voltages are made in a predetermined order and the order must be observed.
FIG. 3 shows a circuit diagram of such a sequence circuit employed for a conventional liquid crystal display unit and a conventional liquid crystal TV set respectively. The sequence circuit generates plural types of voltages from plural types of voltages supplied from a power supply circuit and supplies and stops the supply of those voltages to a liquid crystal display panel of the liquid crystal display unit in a predetermined sequence. In the circuit diagram, units and parts unnecessary for sequence controlling are omitted.
At first, a description will be made for the operation of the sequence circuit when the power to the liquid crystal display unit is turned on. In FIG. 3, upon the turning-on of the power to the liquid crystal display unit and the beginning of voltage supply from the power supply circuit, a high level P-ON-H signal and a high level P-25V-ON signal are inputted to the sequence circuit at a predetermined timing respectively. Consequently, the transistors Q507 and Q511 are turned on and an operation sequence is started by the sequence circuit.
The sequence circuit, when inputting 3.3V, outputs the 3.3V as is to the liquid crystal display panel (sequence 1). Then, −8V is applied to the emitter of the transistor Q513 in the sequence circuit. Thus a potential difference is generated in the base-emitter line of the transistor Q513, thereby the transistor Q513 is self-biased and turned on (sequence 2). Consequently, −8V is supplied to the resistor R517 through the emitter-collector line of the transistor Q513 and −6V is supplied to the liquid crystal display panel through the resistor R517.
And −8V is also supplied to the base of the transistor Q509 through the resistor R517, the Zener diode D519, and the resistor R518. At this time, 40V supplied from the power supply circuit is applied to the emitter of the transistor Q509. Here, the collector terminal of the transistor Q507 is connected to the other terminal of the R511 connected to the base of the transistor Q509 in parallel to the resistor R518. And the emitter of the transistor Q507 is grounded and the transistor Q507 is turned on with a P-ON-H signal inputted to its base. The P-ON-H signal is a control signal that enters the high level (predetermined voltage) when the power to the liquid crystal display unit is turned on and enters the low level (0V) when the power is turned off.
Thus the transistor Q509 is turned on (sequence 3) and a voltage is supplied to the bases of the transistors Q505 and Q505′ from the 40V line through the resistor R508. At this time, the Zener breakdown occurs in the Zener diode D503, thereby the voltage supplied to the bases of the transistors R505 and R505′ is lowered to a predetermined voltage.
Then, the transistor Q505 is turned on (sequence 4) and the transistor Q505′ is turned on (sequence 5). Thus the voltages supplied from the 13V and 21 V lines are output through the resistors R509 and R534 to the liquid crystal display panel as 10.8V.
On the other hand, 40V is applied to the emitter of the transistor R502 through the resistor R552 and the base of the Q502 is grounded through the Q511, thereby the transistor R502 is turned on. Then, 40V is applied to the base of the transistor Q501 through the resistor 552, the emitter-collector line of the transistor Q502, and the resistor R505, thereby the transistor Q501 is turned on (sequence 6). Consequently, 25V is output to the liquid crystal display panel through the emitter-collector line of the transistor Q501.
Next, a description will be made for the operation of the sequence circuit upon the turning-off of the liquid crystal display unit with reference to FIG. 3. Upon the turning-off of the power to the liquid crystal display unit, the voltage supply to the liquid crystal display panel is stopped in the following sequence shown in FIG. 3.
Upon the turning-off of the liquid crystal display unit, the P-25V-ON signal applied to the base of the transistor Q511 enters the low level (0V) and the transistor Q511 is turned off. Consequently, the transistor Q502 is turned off, then the transistor Q501 is turned off (sequence 11). As a result, the 25V supply to the liquid crystal display panel is stopped.
After that, the P-ON-H signal applied to the base of the transistor Q507 enters the low level (0V) and the transistor Q507 is turned off. However, because a current keeps flowing through the resistor Q518, a potential difference is generated between the base and the emitter of the Q509, so that the Q509 is not turned off. Consequently, 10.8V is kept output to the liquid crystal display panel.
After that, upon the stop of the voltage supply including 3.3V from the power supply circuit, the transistor Q510 is turned off, then the transistor Q513 is turned off (sequence 13). At the same time, the 40V supply is also stopped while a voltage is kept applied to the R518 until the remaining 40V charge is discharged completely. Thus the transistor Q509 is not turned off immediately at this time. After the discharging is completed, the transistor Q509 is turned off, then the transistors Q505 and Q505′ are turned off (sequence 12), thereby the 10.8V supply to the liquid crystal display panel is stopped.
Japanese Unexamined Patent Application Publication (JP-A) No. 331927/2005 discloses a display unit power-down short-circuit, in which if the main supply voltage is lowered under a predetermined level, the short-circuit control terminal is connected to a display power line through a diode, thereby the short-circuit is turned on and the display voltage is discharged to the ground.
JP-A-122311/2003 discloses a discharging unit of a picture display panel, in which a voltage dropping means makes trailing of a potential of a common electrode delay than trailing of a potential of a gate voltage.
In the conventional sequence circuit described above, the transistor Q509 is not turned off while the transistor Q507 is turned off in a sequence for stopping the voltage supply. Furthermore, although the voltage supply must be stopped in the order of 11, 12, and 13, the power supply is stopped in the order of 11, 13, and 12. This causes a voltage to be kept applied to the resistor R518 until the sequence 13 is ended and the transistor Q509 keeps outputting a voltage, thereby generating white-line-like noise on the screen. JP-A-331927/2005 and JP-A-122311/2003 are not related to supply of voltages from plural power supplies in a predetermined sequence.